1. Field of the Invention
This invention relates to linear-beam cavity circuits such as coupled-cavity traveling wave tubes (TWTs) and klystrons, and more particularly to the use of RF-lossy dielectric materials in such circuits to provide an improved frequency response of signal amplification.
2. Description of the Related Art
Linear-beam circuits such as TWTs and klystrons cause a stream of electrons to interact with a radio frequency (RF) electromagnetic field in a manner that amplifies the electromagnetic field energy. In a TWT, for example an electromagnetic wave is propagated along a slow-wave circuit, such as a conductive helix wound around the path of the electron stream or a folded waveguide type of structure in which a waveguide is effectively wound back and forth across the path of the electrons. The slow-wave circuit provides a propagation path for the electromagnetic wave that is considerably longer than the axial length of the circuit, so that the traveling wave may be made to effectively propagate at nearly the velocity of the electron beam. The interactions between the electrons in the beam and the traveling wave cause velocity modulations and bunching of the beam electrons. The net result is a transfer of energy from the electron beam to the wave that is traveling along the slow-wave circuit.
The main components of a conventional TWT 2 are illustrated in FIG. 1. An electron gun 4 generates and feeds an electron beam into a slow-wave structure 6. The electron beam is guided through the slow wave structure by a static magnetic focusing field, and is captured at the opposite end of the slow-wave structure 6 by an electron collector 8. The electromagnetic wave is fed into one end of the slow-wave structure through an RF input coupler 10, and is coupled out from the opposite end of the slow-wave structure through an RF output coupler 12. TWTs are commonly used to provide a high degree of signal amplification at microwave and millimeter wave frequencies for communications, radar and other applications.
TWTs and other linear-beam tubes such as klystrons are designed to operate over a given frequency band, such as 3.1-3.5 GHz. However, conventional devices exhibit a non-uniform amplification response at different frequencies within their nominal operating bands, and coupled-cavity TWTs are also subject to oscillations at various cut-offs of the frequency bands in which the circuit can propagate an RF wave. In particular, TWTs tend to be unstable at the high frequency cut-off of the lowest passband, which contains the operating band. In an effort to improve the frequency response and to provide stability, RF-lossy ceramic "loss buttons" have been distributed around the internal cavity periphery of a TWT. The loss buttons are formed from a ceramic material such as BeO or MgO that is mixed or doped with a conductive material, typically SiC. The loss buttons are typically cylindrical, with their axes parallel to the TWT axis and lodged in the tube wall.
To smooth out the TWT's frequency response, "reentrant loss buttons" have been used. These buttons project into the circuit cavities from the wall, and introduce a loss element by disrupting the RF wave within the tube. Non-reentrant, or tangent, loss buttons are also typically used, in which the edge of the button lies along a tangent to the inner cavity wall. The function of non-reentrant loss buttons, is to add loss over a narrow frequency range, such as near the upper cut-off frequency of the passband. This can be effectively used to eliminate upper cut-off instability, but it is not designed to smooth out the tube's amplification response over the full operating frequency band.
Non-reentrant loss buttons are doped with a relatively low level of SiC, typically 1-5%, as opposed to the typical 15% or greater doping for reentrant buttons used to smooth the frequency response over the operating band. The use of reentrant and non-reentrant loss buttons are described in U.S. Pat. Nos. 3,602,766 to Grant and 3,221,204 to Hant et al., respectively; both U.S. patents are assigned to Hughes Aircraft Company, the assignee of the present invention. Coupled-cavity TWTs and klystrons are described in general in A.S. Gilmour, Jr., Microwave Tubes, Artech House, Inc., 1986, pages 201-209 and 302-313.
Both reentrant and non-reentrant loss buttons typically have diameters equal to about half the field wavelength within the button for the frequencies they are designed to operate at. Since the conducting tube wall within which the loss button is positioned shorts out the parallel field component at the wall, and since the major electric field component in the button is the component in the direction of the beam axis, which is parallel to the button axis, the field at opposite ends of the button's diameter will typically be at a low or zero value. With a diameter approximately equal to half a wavelength of the field in the button, a resonant condition is established. Non-reentrant buttons, being made of material with a low percentage of lossy component, are highly frequency selective, with a high Q factor. They can provide significant loss over a narrow frequency range. Reentrant buttons, made of material with a high percentage of lossy component, have a more broad and shallow loss response.
While reentrant loss buttons are effective in smoothing out the tube's frequency response, they tend to disturb the circuit "match" at the ends of the tube by giving rise to internal cavity reflections. Such mismatches contribute to ripple in the amplification response, counteracting the smoothing effect of the loss. In practice, tubes with re-entrant buttons require a greater effort in manufacture to achieve acceptable matches. Furthermore, they are not particularly efficient in terms of the amount of loss they introduce for a given button size and mass. Non-reentrant loss buttons can provide a large amount of loss at the cut-off region, but they are not effective in smoothing out the frequency response over the operating band.
An alternate approach to smoothing the frequency response is to provide an RF-loss coating along the tube's inner cavity walls. This technique is described in U.S. Pat. No. 3,453,491, also assigned to Hughes Aircraft Company. However, this technique is also less than ideal. More complex processing is required than the processing used for loss buttons, and the amount of loss that the coatings can provide is relatively limited.
In broadband klystrons, the cavities between the input cavity and the output cavity are designed to resonate at specific frequencies in or near the operating band, and the resonances have specified widths (values of cavity Q) for optimum broadband response. The cavity Q may be controlled by inclusion of RF loss, using loss coating or lossy ceramic elements as in a coupled-cavity circuit.